WirelessMoves

How much is -90 dBm?

When talking about signal strengths, values are usually expressed in dBm, which is, according to Wikipedia 'an abbreviation for the power ratio in decibels  (dB) of the measured power referenced to one milliwatt (mW)'. In other words, dBm is a value for the power or signal strength on a logarithmic scale.

In practice, signal strengths between -60 and -90 dBm are encountered at the receive antenna of a mobile device. So how much is that actually? On the high end of the scale, -60 dBm equals 0.000000001 Watts, while – 90 dBm are 0.000000000001 Watts.

I find two things quite remarkable in these numbers: First, both values are very small, definitely not enough to power anything and its amazing that receivers can pick up such small signals. And secondly, the wide range a receiver has to adapt to as the weakest signal compared to the strongest signal in this example is 1000 times weaker.

Three Austria Reports 700 TB per Month

Interesting news from mobile network operator "3" in Austria: Heise reports in this article that their monthly data volume is now at 700 Terabytes per month. According to Wikipedia they currently have about 1 million subscribers which makes this number even more interesting, as on average that would be 700 MB  per subscriber per month. Of course many of their subscribers only use the network for voice calls and SMS so imagine what the number must be for the typical mobile Internet user in their network. With Austria being a super competitive mobile network country and usage numbers way ahead of those in other countries it's a good indicator of what is possible and of things to come.

Will the Clicking be Back with CPC?

One cool feature of UMTS compared to GSM is that you can't hear the interaction with the network when the mobile device is close to a radio or telephone conference equipment. This is because while GSM transmits bursts whose transmit on/off changes can be picked up by such equipment, UMTS transmits continuously in the uplink direction and as a consequence no annoying clicking sounds can be heard. With the introduction of Continued Packet Connectivity (CPC), however, this will change to conserve power. So I am wondering if CPC devices will again produce some sort of a clicking noise in other audio equipment!? I hope not... And in case it does it would be nice if CPC could be deactivated in the device. I know, highly unlikely that such an option will be put into devices but then one never knows. If a 'sexy' name can be attached to it someone might just do it. How about 'stealth mode' or 'undisturbed meeting mode'. I am sure marketing departments can come up with something better.

AMR Half Rate in New York

I was in New York for a couple of days recently and as my N95-8GB (yes, I am a traditionalist) can only do UMTS 2100 MHz my voice calls and small screen Internet connectivity had to be content with the GSM network layer. But there are interesting things to discover here as well.

It looks like AT&T consistently uses AMR Half-Rate in the center of the city to double their voice capacity. All calls I established were always set-up with this codec. Also, AT&T uses both the 850 and 1900 MHz bands and if the mobile detects both, the network always handed the voice call over to a 1900 MHz carrier, even when the signal in the 850 MHz band was 30 db stronger. That could have many reasons, maybe they use the 1900 MHz band as a capacity layer and reserve the 850 MHz carriers for difficult terrain and indoor connections.

Speaking of difficult terrain: AT&T when you have a minute, have a radio team look at your coverage in Penn Station. Even when standing still, my calls in the underground station frequently dropped. Despite a high signal level, audio quality was horrible, which points to interference, and every couple of seconds, the call was handed over between the 850 and 1900 MHz layer and the traffic channel bounced between AMR Full-Rate and Half-Rate. Quite a frustrating experience.

Internet Over The Clouds

What I really miss on long-haul flights is Internet connectivity. A couple of years back, Boeing and its Connexion system was a cool option if you flew Lufthansa and a few other carriers but it seems the service was not profitable and Motorola discontinued the service. Too bad, long-haul is offline again since then. But it looks like things are going to change again, as Lufthansa has announced they will enter a cooperation with Panasonic and offer Internet connectivity again. Few details were available so far but now Heise has published an interesting article on the topic giving some details on the system and the general renewed interest in in-flight Internet and GSM connectivity. The original article is here and the Google translated version to English is here.

Like the earlier system, the Panasonic equipment is capable of downlink speeds in the double digit megabit per second range which should be sufficient even if a couple of dozen people are online at the same time. For the satellite connection the Ku-band (10-12 GHz) is used. In addition to Panasonic, there are two other companies in the business. OnAir uses a narrowband signal and the Heise article says the speed is limited to 400 kbit/s. That's too slow for full screen Internet connectivity in my opinion so maybe OnAir focuses more on voice calls, SMS and small screen (smartphone) connected applications. And then there is ROW44 with similar speeds as Panasonic but so far limited to the US market.

All very good signs that in-flight Internet might have a revival and personally, I hope it will be sooner rather than later.

VoIP In an Overloaded Network

It is always said that Quality of Service (QoS) mechanisms are important for VoIP, but what does it mean in practice? Well, as long as that Skype connection just works it doesn't really mean a whole lot. It suddenly means a lot though, when the call breaks up and the other end is barely understandable due to congestion that provokes rapid round trip time changes between 300 ms and 1600 ms in the course of just a few seconds.

This happened to me recently and at the time I wished I had some QoS preferring my VoIP packets over whatever else the other 99 participants of the meeting were transferring over that Wi-Fi link connected to a way too slow backhaul link at the time. It suddenly makes you aware of the benefits of network enforced QoS. And it must be the network because otherwise half the participants would immediately tweak their PCs to demand a better QoS for their packets irrespective of the application. Well, that's at least what I would do...

So in the end I resorted to making a call over GSM. Just another way of getting QoS, really, just not on an IP bearer.

The Cable Mouse Is Back

Yes, I very much like my wireless mouse but I have just discovered the use for a cable bound mouse these days again: On an airplane! For long haul flights, if you have enough space to use a mouse it comes in very handy as wireless mice are not allowed to be used.

I wonder if that "suddenly" changes again once airlines introduce Internet access on some of their flights again (e.g. Lufthansa via satellite or American carriers via land based systems) but for now, the cabled mouse comes in quite handy.

My Personal Signaling Load

With the rising use of the mobile Internet, the amount of signaling required to set-up and maintain radio bearers is changing. While in pre-mobile Internet times, radio and core networks were mainly dealing with location updates and signaling due to SMS and voice calls, always-on smartphones are used quite differently.  Lets do a little comparison:

Someone who's only two applications that require network interaction are SMS messages and voice calls might have the following signaling pattern:

• 4 radio bearer setups a day for incoming SMS messages
• 4 radio bearer setups a day for outgoing SMS messages
• 2 outgoing voice calls (some with handovers, which increase signaling load)
• 2 incoming voice calls (some with handovers, which increase signaling load)
• 5 location updates a day (periodic and some triggered because the user as he moves)
• no routing area updates since the mobile is only attached to the circuit switched side of the network
• These numbers are obviously for someone who doesn't move a lot and who is, by today's definition, someone with a low usage. The young generation often sends and receives my more SMS messages a day.

Personally, my CS messaging load is a bit different. I send and receive fewer SMS messages as I mostly use e-mail for text messaging as my friends are distributed throughout the world and these exchanges are usually also not required to be had in real time. But I usually make and receive more than two calls. I try to minimize calls when moving as I find it impolite to talk on the phone while on a public transport and also prefer some privacy when calling.  But for the calls I do make while on public transport I am glad handovers exist.

But now lets go to the packet switched side and see how things are here:

• I am always-on and receive about 4 e-mails per hour. In other words, there are 4 signaling evens per hour or about 4 * 15 hours (per day) = 60 events per day for this application alone. That easily surpasses all my CS signaling
• While on public transportation I often use the time to read my favorite blogs and browse my favorite news web sites. Let's say I browse on 30 pages a day and read each long enough for the radio link to go back to idle. In other words, 30 signaling events.
• While I am moving the radio link has to be maintained and handed over to neighboring cells which creates quite a significant amount of signaling, especially with my 'old' N95 that doesn't have fast dormancy and hence the network needs to work more to maintain the established but not really required radio bearer after background push/pull requests.
• Also during quick breaks I try to catch a news bite or two to know what's going on in the world. Lets say that's an extra 10 signaling events per day.
• No instant messaging or other push applications are running in the background but more and more people are doing that these days, too.

In total that's easily 100 signaling events on the PS side per day. Compared to the 15 signaling events on the CS side for voice and SMS, that's quite a difference, not only for the network but also for the mobile, i.e. the impact on the battery charging interval is significant.

3G Coverage in France

Today I've got something positive to say about network coverage in France: While the country has been slower than most other European countries to deploy 3G in areas other than big cities, it seems coverage has much improved in recent years. Over the weekend I was in a remote village near Nantes and experienced great 3G coverage. Quite unexpected! And then on Sunday, a 6 minute call out of a high speed train darting through the countryside at 300 km/h on GSM without a call drop. Very good! Now some more prepaid love for Internet access and I'm a happy customer!

LTE Voice Takes A Break...

Inspired by this post over at the 3G4G wireless blog on VoLGA and VoLTE architecture differences, I reflected a bit on why it has become a bit silent around LTE voice recently. From a VoLGA point of view, maybe it is because there is little more it can prove until the time real LTE mobile devices (not USB sticks) come to the market.

Back in February 2010, VoLGA was demonstrated at the Mobile World Congress in Barcelona as fully functional, using live network equipment. It's here and it's working but until real devices become available, there is nothing more to be shown that could go beyond it. And with closely related UMA (Unlicensed Mobile Access) stacks used in practice today on Nokia, RIM and Samsung devices for Voice over Wi-Fi, I have little doubt that this final piece can be implemented very quickly. Same thing for VoLTE and CS Fallback, they are there on paper and are waiting for devices they can be implemented on.

But once the solutions can be demonstrated in practice, I expect that the competition between them will accelerate again. It's going to be interesting to see if the simplicity, full functionality from day one, fast call setup times and a relatively simple but hyper-important in-call handover from LTE to GSM / UMTS of VoLGA will trump over other solutions.